HVAC Issue

[Articus]

Hi,

2001 A4. I've got an issue where heat blows weakly through the HVAC vents at all fan speed settings. Same thing when I set to blow only in footwell, only Center, only windshield blower. Windows would not defrost this morning. Its like the fan speed is stuck on low. It makes a whistling sound that gets louder as I increase fan speed with the controls, but doesn't actually blow any harder.
-I've already tested the blower motor by pulling it out of the HVAC and seeing it spin and ramp up speed. I also tested a spare blower motor outside of the HVAC and both work fine so I don't think it's the blower motor unless both are faulty.

During the summer, the A/C is cold so don't think it's related to compressor.

When I switch the HVAC to Recirculation mode (only works when not using defroster), everything blows hard, hot, and at full speed like it should. Of course the only thing that still doesn't work is the windshield defrost because it shuts off recirc mode.

Does anyone know what I need to fix/replace? What changes with the flaps inside the HVAC when in recirc mode?

[Articus]

.?

[RENOxDECEPTION]

clogged air filter.

because recirc doesn't filter the air.

[Beth641]

clogged air filter.

because recirc doesn't filter the air.

should he change the air filter?

[RENOxDECEPTION]

...yes...

[Articus]

I last changed this cabin air filter this April. I took it out to look at it anyway, no signs of really being blocked or dirty. Just some stray bits of nature in it that came out when I turned it upside down.
Today, I was running the heat to the center vents only and it was blowing like normal. When I switch to defrost mode, I find the air flow to be weak though there. Can I get anything from vagcom?